The present invention relates to disc drives, and more particularly to spindle motors in disc drives which use hybrid ceramic ball bearings.
Spindle motors in disc drives typically have a set of electromagnets fixed to a shaft and a set of permanent magnets attached to a rotatable portion referred to as the housing. The electromagnets are arranged in a circle surrounding the shaft. As the electromagnets are energized in a sequential pattern travelling around the shaft, the permanent magnets are induced to follow, thus imparting a rotational force to the housing. The shaft and the housing are connected by bearings to allow for the rotation of the housing about the shaft.
An inner ring of the bearing is mounted to the shaft and an outer ring of the bearing is fixed to the housing. In this assembly, an inner raceway of the inner ring faces an outer raceway of the outer ring to form a cavity within which rolling elements, for example, balls, are located. A preload is applied to the bearings to establish proper contact between the rolling elements and the inner and outer raceways.
Bearings used in spindle motors typically use balls and raceways made from steel. Other types of bearings, which may be used in higher speed spindle motors, are hybrid ceramic bearings. These typically have raceways made of steel and balls made of ceramics. However, within a typical operational temperature increase of a disc drive, the steel raceways expand faster and to a greater extent than the ceramic balls. This differential expansion results in a decrease in the preload on the bearings, which means that the ceramic balls are no longer in proper contact with the raceways, so the performance of the spindle motor deteriorates.
However, there are advantages associated with the use of hybrid ceramic bearings in spindle motors, and some of these are briefly described below.
Compared to steel balls, ceramic balls are about forty percent less dense, and therefore have lower acoustics or natural frequencies.
Ceramic balls have a longer life due to lower heat generation between the contact surfaces. xe2x80x9cCold weldxe2x80x9d spots or micro welding which may occur with steel on steel is less likely to happen with the use of dissimilar materials. Therefore the use of ceramic balls with steel raceways reduces the fretting and Hertzian stress on the raceways, hence increasing fatigue life.
Ceramics have a higher modulus of elasticity in comparison with steel, and for a given size bearing, ceramic bearings have two times the bearing stiffness of steel bearings.
It has been found that the use of ceramic balls in bearings contributes to increased grease life. This is because there is less or no metal particle generation from wear, and because of the lower heat generation when ceramic bearings are used.
Because silicone nitride is a different material from that of the raceways, the balls offer a non-galling advantage.
Another advantage of using ceramic balls is that, owing to the nature of silicon nitride, the surface finish potential of ceramic balls is better than what can be achieved with steel balls.
There is therefore a need to overcome the problem of losing the preload on hybrid ceramic bearings so that such bearings can be utilized in spindle motors for disc drives. The following description will make clear how the present invention provides an improved solution to this and other problems.
According to one embodiment of the invention, a spindle motor for use in a disc drive has an upper inner bearing support axially displaced from an upper outer bearing support by a first axial displacement, as well as a lower inner bearing support axially displaced from a lower outer bearing support by a second axial displacement. The spindle motor includes a shaft which is operably connected to the upper inner bearing support and to the lower inner bearing support. The spindle motor also includes a housing which is operably connected to the upper outer bearing support and to the lower outer bearing support. The shaft and the housing are configured to expand axially by different amounts for an increase in temperature such that the first axial displacement and the second axial displacements are increased over the increase in temperature.
The spindle motor includes rolling elements located between the upper outer bearing support and the upper inner bearing support. The spindle motor also includes rolling elements located between the lower outer bearing support and the lower inner bearing support. The rolling elements if ceramic, are thus configured to expand slower and to a lesser extent than the upper bearing supports and the lower bearing supports for the increase in temperature. Hence, an increase in temperature will tend to loosen the fit between the upper bearing supports and the rolling elements located therein, as well as loosen the fit between the lower bearing supports and the rolling elements located therein. The shaft and the housing are purposely configured for differential expansion which results in an increase in the first axial displacement and in the second axial displacement, thereby offsetting any differential expansion between the bearing supports and the rolling elements.
In one preferred embodiment of the present invention, the upper and lower bearing supports are made essentially of steel and the rolling ball elements are made of ceramic. The shaft is made of steel and the housing includes at least one aluminum component arranged between upper bearings and lower bearings.